1. Field of the Invention
This invention relates to a D/A converter circuit using resistor strings.
2. Description of the Related Art
An R-2R type D/A converter circuit has been known as a D/A converter circuit using ladder resistors. This type of D/A converter circuit is provided with the R-2R ladder resistors to which a reference voltage VREF is supplied, (n+1) constant current sources each corresponding to each of (n+1) bits (ranging from LSB (Least Significant Bit) to MSB (Most Significant Bit)) in a digital signal, respectively, and (n+1) switches S0-Sn each corresponding to each of the (n+1) bits in the digital signal, respectively, as shown in FIG. 9, and is capable of converting the (n+1) bit digital signal into an analog signal by turning the switches S0-Sn on/off. This type of D/A converter circuit is disclosed in Japanese Patent Application Publication No. H10-135836.
When data of the (n+1) bit digital signal varies, the switches S0-Sn are turned on or off accordingly, and corresponding currents flow from the constant current sources to the R-2R ladder resistors in the circuit described above. At that time, however, there is caused a slight fluctuation in a voltage (the reference voltage VREF) at a node A shown in FIG. 9. In addition, there are caused slight variations in ratios among constant currents I0-In drawn from the (n+1) constant current sources that are made of current mirrors. As a result, there is caused a “bit inversion” in a six-bit or more than six-bit D/A converter circuit, as shown in FIG. 10. The bit inversion means a drop in an analog output voltage as a value of the digital signal increases.
Also, there has been known a D/A converter circuit using a resistor string as shown in FIG. 11. It is a nine-bit D/A converter circuit provided with a resistor string composed of 511 serially connected resistors to which a reference voltage VREF is supplied and 512 switches S1-S512 and capable of converting a nine-bit digital signal into an analog signal by turning the switches S1-S512 on/off corresponding to the nine-bit digital signal. Although the bit inversion does not occur in the D/A conversion circuit using the resistor string, there is a problem that it takes a very large layout area (as large as five times of a layout area required by the R-2R type D/A converter circuit in the case of the nine-bit D/A converter circuit).
This invention is directed to offering a multi-bit (six-bit or more than six-bit) D/A converter circuit that is capable of preventing the bit inversion as well as reducing the layout area.